In a wireless communications system, data communication is performed between devices by sending and receiving a signal. Before data communication is performed, a receiving device needs to perform data frame synchronization with a sending device. To implement the data frame synchronization between the receiving device and the sending device, the sending device needs to send a synchronization signal. The receiving device implements frequency synchronization and time synchronization by detecting the known synchronization signal. Afterward, the receiving device further needs to periodically detect a synchronization signal, to maintain the frequency synchronization and the time synchronization. Otherwise, the data frame synchronization may be lost in a subsequent data communication process.
In a Long Term Evolution (LTE) system with a licensed carrier, by means of coarse synchronization, a frequency domain deviation is within a half subcarrier width, and an approximate range of time synchronization is determined, and by means of fine synchronization, the frequency domain deviation is further reduced, and an arrival time of a first path is accurately determined in order to complete a whole synchronization process. The coarse synchronization is implemented using a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). The fine synchronization is implemented using a cell-specific reference signal (CRS). It is generally considered that in an LTE system, a time in which a receiver needs to obtain again synchronization is 40 milliseconds (ms), and a period in which the receiver needs to continuously trace the synchronization is 5 ms. In the LTE system with a licensed carrier, the system may continuously occupy a carrier, and therefore, the system may send synchronization signals (a coarse synchronization signal and a fine synchronization signal) according to a synchronization requirement. In the LTE system, a signal for coarse synchronization is sent at an interval of 5 ms. A signal for fine synchronization is sent at an interval of 0.285 ms.
However, on an unlicensed carrier, a time in which a system occupies the unlicensed carrier is neither continuous nor periodic because the unlicensed carrier is accessed in a contention manner. Consequently, the system cannot ensure that a signal for synchronization can be sent continuously and periodically. Therefore, a new data frame synchronization method needs to be designed for an unlicensed carrier in order to implement data frame synchronization on the unlicensed carrier.